I just took my first road trip with the Epic. I started with my current location set via GPS and I had Network assist on.
I used Sprint Navigation and Google Navigation as my test programs.
My static location was around 100-200 yards from my actual location. Comparing to my Treo Pro, this is bad since the TP put me right on my deck.
I then took a ride around the neighborhood. I have navigation in my vehicle as well. The Epic was slow to update and lagged as much as a football length behind while in motion. As I slowed up, it caught up until I moved forward again. Again. on the same route, the Treo Pro worked nearly as well as the in vehicle navigation. It did falter a little going under a bridge, but it caught up quickly, much faster than the the Epic. It should be noted that the Treo was never known for it's GPS performance.
I can only conclude that there may be something to the GPS issues on the entire Galaxy line. Some may conclude that the differences are marginal. I submit that there is an expectation that is being met by some phones that others do not quite match up to.